1. Field of the Invention
This invention addresses the art of manufacture of composite structural parts comprised of oriented structural yarns presented in a cured polymer matrix. More particularly, this invention deals with the curing of such materials, in a high speed and accurate fashion.
2. Background of the Prior Art
Oriented structural yarn/polymer matrix composites are receiving increasing attention in the work place in applications calling for high strength, good temperature resistance, good corrosion resistance, and at the same time, reduced weight and therefore energy savings. See, generally, Chou et al, Composites, Scientific American, October 1986, 193-203. These structural hybrid materials will constitute an increasingly large, and important, portion of the aerospace technology of this country in the foreseeable future, and contribute heavily to critical industries including the military.
In general, oriented structural yarn/polymer matrix composites can be characterized as comprising at least one layer, usually a plurality of layers, of structural yarns, the yarns being oriented in a precise direction. It is frequent to have differing orientations in varying layers, although, for the most part, in any given layer, the orientation of all yarns is substantially similar. This is to enhance load distribution, and greatly increases the overall strength of the composite part.
Structural yarns can be thought of as those yarns having a high modulus of elasticity (Young's), in excess of about 8-10 million. Certain forms of glass fibers, most carbon yarns, boron yarns and the like all fall in this class. Other members of the class, without limitation, include polyamides, and within that, such polyaramides as Kevlar.TM.. Hybrid yarns are also known, and occasionally, lower modulus, but exceedingly high tensile strength yarns, such as nylon, may be employed for structural purposes.
The plurality of layers can be held together, initially, by some sort of adhesive or stitching. Eventually, these are saturated with, and surrounded by a polymer matrix, which is cured, to provide a high strength, lightweight article. The majority of resins currently employed for this purpose are thermosetting resins, and exemplary among them may be epoxys, polyesters and the like. Certain resins, including those designated in Peek, are thermoplastic, and because of their outstanding temperature performance, are receiving increased attention in the work place.
In addition to their highly desirable physical characteristics, these products offer enhanced potential in parts consolidation and high strength to weight ratio. However, the composites currently available remain expensive, particularly in such exacting technologies as the aerospace market. One reason for this high cost is the continued high cost of processing materials, particularly, the high cost involved in transforming the resin-saturated uncured intermediate into a cured, stiff and strong product. Whatever process is employed for this transformation must be relatively rapid to reduce cost and enhance deliverability, but, at the same time, ensure accurate, precise, repeatable molding, such that the exceedingly narrow tolerances of the aerospace market are met.
Conventional methods for curing these composite articles include autoclaving, and compression molding. Neither is ideal, for a variety of reasons. Compression molding is capital intensive. It is also limited to the production of short articles, and cannot be used to make articles greater than about 8 feet long. On the other hand, autoclaving tends to be a much slower process. Frequently, curing cycles, from initial start up to the end of cool down, exceed six hours and more. To overcome this problem, it is frequently the practice to cure several articles in a single autoclave cycling. The problem encountered is the fact that, whatever type of processing controls are maintained, they tend to be based on a "worst case" of the characteristics of the individual parts being processed, at best, and, due to the fact that the heat transference is through the air, the process is extremely energy intensive at higher speed. Thus, the critical need for accuracy is difficult to achieve.